The present invention relates to apparatus for electrostatically recording X-ray images and, more particularly, to a novel device utilizing a mesh-base photocathode photoelectron discriminator means for reducing background noise and obtaining greater contrast in such apparatus.
Apparatus for electrostatically recording X-ray images, particularly a device of the type having an air-exposable recording film which may rapidly be processed by xerographic techniques, is highly desirable by users of X-ray technology. Apparatus fulfilling these requirements is disclosed in U.S. Pat. No. 3,940,620, issued Feb. 24, 1976 and assigned to the assignee of the present invention, which discloses an electrostatic X-ray image recording device comprised of two spaced electrodes with a gas-filled gap therebetween. One of the electrodes comprises a layer of an ultraviolet emitting fluorescent material and a layer of an air-exposable ultraviolet-sensitive photoemitting material thereon. A plastic sheet is positioned adjacent to the remaining electrode and an electric field is applied across the gap to accelerate photoelectrons, emitted by the photoemitting material responsive to incident vacuum-ultraviolet (VUV) photons responsive to incident X-rays, to the plastic sheet. The electrostatic image formed on the plastic sheet is then developed by xerographic techniques after exposure.
The gap between the pair of electrodes is filled with a gas which, when struck by the accelerated photoelectrons moving across the gap, generates ion-electron pairs causing amplification by an avalanche effect in the gap.
The photoemitting material is stimulated to emit electrons not only by the desired VUV photons, but also responsive to direct stimulation by X-rays passing through the phosphor layer. The direct X-ray-stimulated photoemission produces fast electrons, i.e., electrons of sufficient kinetic energy to generate secondary ion-electron pairs through ionization of the gas contained in the gap. The secondary electrons are amplified by the gas, along with the desired slow electrons, to create sufficient "noise" to reduce the available contrast ratio of electrostatic charge on the imaging sheet. The gas in the gap is normally substantially at atmospheric pressure.
One method for reducing the contribution to the "noise" due to fast electrons is to reduce the pressure of the gas in the gap whereby fewer gas molecules are available for ionization. This arrangement complicates the structure of the device as a differential pressure-sealing envelope must be provided between the electrodes and about the periphery of the gap to allow reduction of the gas pressure, and requires additional pressure-reduction means, such as a vacuum pump and the like. Further, the bulky differential-pressure-sealing means must be removed from the device to allow removal of the plastic film for subsequent development, which time-consuming task negatives one of the the major efficiency factors of the air-exposable film-electrode combination.